This application claims benefit of Japanese Patent Application No. 2002-364577 filed on Dec. 17, 2002, the contents of which are incorporated by the reference.
The present invention relates to transmission power control systems and, more particularly, to outer loop transmission power control systems in mobile wireless communication in W-CDMA (Wideband Code Division Multi Access) system.
In outer loop transmission power control in the W-CDMA system, a mobile station (or a base station) measures such communication quality as BLER (block error ratio) and BER (bit error ratio), and controls desired SIR (desired signal power versus noise power ratio) from the comparison result of the communication quality with the desired communication quality. It is thus possible to indirectly control the transmission power of the base station and obtain desired communication quality. However, when only the comparison of the communication quality is used as the basis of decision, it may become impossible to obtain proper desired SIR control when the transmission power of the base station settles to the maximum or minimum.
Specifically, in the W-CDMA system all the channels may use the same frequency, and interferences from other communication channels may arise. The extent of interference constitutes a main cause of determining the line capacity, and it is preferred in view of the line capacity to make the transmission power of the base station as low as possible. In the meantime, the communication quality in a mobile unit is dependent on the received SIR. In other words, by increasing the transmission power of the base station, the received SIR is increased, thereby improve the communication quality. It is thus necessary to satisfy the communication quality necessary for services to be utilized and preset the optimum transmission power to minimum transmission power.
In the case of wireless communication, unlike the case of wired communication, changes in the ambient environment or movement of the mobile station (or mobile unit) causes changes in the transmission power of the base station that is necessary for obtaining a constant received SIR. Also, the received SIR necessary for obtaining a constant communication quality is changed.
With the above affairs taken into considerations, in the W-CDMA system the transmission power control system is prescribed such that the line capacity and the communication quality can be optimized. The transmission power control is carried out in two stages. In one method, the desired SIR is preset, and a transmission power control signal is sent out to the base station to make the received SIR to be the desired SIR (inner loop transmission power control). The base station controls the transmission power based on the received control signal. The operation will now be briefly described.
In the case of (received SIR)<(desired SIR): a transmission power increase request control signal is sent out.
In the case of (received SIR)>(desired SIR): a transmission power of reduction request control signal is sent out.
In another method, the communication quality is measured at a constant time interval, and the desired SIR is controlled based on the comparison result with the desired communication quality (outer loop transmission power control). The operation will now be briefly described hereinafter.
In the case when the measured communication quality is inferior to the desired communication quality: the desired SIR is increased.
In the case when the measured communication quality is superior to the desired communication quality: the desired SIR is reduced.
Generally, the outer loop transmission power control is slow in the control speed compared to the inner loop transmission power control. This is so because the communication quality measurement requires more time than the case of the SIR measurement.
General techniques concerning the transmission power control method, apparatus and system in such wireless communication are disclosed in various literatures (for instance, see Literature 1: Japanese patent laid-open No. 2002-18539.8 and Literature 2: WO97/50197).
Problems in the prior art will be described with reference to FIGS. 5 and 6. In these FIGS. 5 and 6, the abscissa is taken for time, and the ordinate is taken for base station transmission power (see (A)), SIR (see (B)) and BLER (C)) overlapped one over another in the mentioned order. As shown in (A), the base station carries out the transmission power control based on a control signal sent out from a mobile unit. However, the maximum and minimum levels of transmission power are preset. In the case when the communication quality is inferior to the desired communication quality while the base station transmission power is maximum, by controlling the desired SIR based on the sole communication quality as in the prior art the desired SIR is increased due to the inferior communication quality.
Meanwhile, since the base station transmission power is maximum, the received SIR is brought to a state without follow-up of the desired SIR (state A in FIG. 5). When the communication environment is suddenly improved from the state A, that is, when an environment appears that the desired communication quality is obtainable even with a lower base station transmission power level than the maximum level. The received SIR follows up the desired SIR at the high level (state B in FIG. 5). Subsequently, owing to the large received SIR the satisfactory communication quality state is continued, and the desired SIR is gradually reduced (state C in FIG. 5). Finally, the received SIR is settled in the proper desired SIR (state D in FIG. 5). In the above process, the excessive transmission power state is continued for time E, leading to a long time until the received SIR is converged to the proper desired SIR, which is undesired in view of the line capacity. This problem stems from the fact that because of the maximum base station transmission power level, despite no increase of the received SIR, the desired SIR is increased with the sole communication quality as the reference of decision.
In the converse case when the communication quality is superior to the desired communication quality although the base station transmission power is minimum in level, the desired SIR is reduced owing to the satisfactory communication quality. On the other hand, the received SIR does not follow up the desired SIR because of the minimum base station transmission power level (state A in FIG. 6). When the communication environment is suddenly deteriorated from the state A, that is, when an environment is brought about that the communication quality is inferior or the communication can not be maintained with the minimum base station transmission power level, the received SIR is suddenly reduced by following up the desired SIR (state B in FIG. 6). Subsequently, due to low received SIR the communication quality is deteriorated to result in increase of the desired SIR and the received SIR (state C in FIG. 6). Finally, the received SIR is settled in the proper desired SIR (state D in FIG. 6).
A problem that is encountered in this circumstances is the reduction of SIR in the state B. The desired SIR increase control is made after decision that the communication quality is inferior, and it is thus lower in speed than that speed of follow-up of the desired SIR by the received SIR. Therefore, it is possible that the communication quality is deteriorated to break the communication before increasing the desired SIR by deciding that the communication quality is inferior. This occurs due to the fact that in the state A in FIG. 6 the desired SIR is received despite the failure of follow-up of the desired SIR by the received SIR.